1. Field of the invention
This invention pertains to a matching circuit handling multiple bands which, in a plurality of frequency bands, establishes matching between circuits having different impedances. It pertains to matching circuits built into small-sized multiband power amplifiers which amplify, with high efficiency, signals in a plurality of frequency bands used e.g. in mobile communications and satellite communications.
2. Description of Related Art
Accompanying the diversification of services offered by means of radio communications, conversion to multiband capability for processing signals in a plurality of frequency bands is required of radio equipment. As an indispensable device included in radio equipment, there is the power amplifier. In order to carry out efficient amplification, there is a need to obtain impedance matching between the amplification element and its peripheral circuits, so a matching circuit is used. As an example of a conventional multiband power amplifier, technology as shown in Reference 1 (NTT DoCoMo Technical Journal, Vol. 10, No. 1: “Mobile Handsets”) is disclosed.
The configuration of the 800 MHz/2 GHz band power amplifier shown in Reference 1 is shown in FIG. 1, and the operation thereof will be explained. The transmitted signal coming from the transmitter is input into the single pole terminal of an input switch 150, a Single Pole Double Throw (SPDT) switch. Next, the transmitted signal, by being switched by input switch 150, is input into an 800 MHz band amplifier 151 connected to a double throw terminal of input switch 150, or a 2 GHz band amplifier 152. The output signals of 800 MHz band amplifier 151 and 2 GHz band amplifier 152 are switched by an output switch 153, a Single Pole Double Throw switch, and supplied to an antenna.
In FIG. 2, the configuration of 800 MHz band amplifier 151 and 2 GHz band amplifier 152 is shown. Each amplifier is configured with a series connection of an input matching circuit 160, an amplification element 161, and an output matching circuit 162. Input matching circuit 160 obtains matching between a signal source 163, whose output impedance does not depend on the frequency, and amplification element 161. Output matching circuit 162 obtains matching between the output impedance of amplification element 161 and a load 164.
Since the input impedance of amplification element 161 constituting each amplifier varies with frequency, input matching circuit 160 and output matching circuit 162 are different depending on the operation frequencies, even if the same amplification element 161 is used. Accordingly, as shown in FIG. 1, separate amplifiers 151, 152 handling each frequency band have been necessary. Consequently, there has been the problem that the total circuit area of the transmitter became larger as the operating frequency bands rose.
In order not to increase the circuit area of an amplifier, the method of designing matching circuits for wideband operation can also be considered. However, compared to matching circuits designed for narrowband operation, the result is that there occurs a reduction in gain and efficiency. Accordingly, with respect to these problems, the applicant of the present application first proposed, in Reference 2 (International Publication No. WO 2004/082138 Pamphlet), a matching circuit which can handle the conversion to multiband capability. The input matching circuit of the amplifier disclosed in Reference 2 is shown in FIG. 3. E.g., the FET (Field Effect Transistor) input impedance can be expressed as a load 170 (impedance ZL(f)) having frequency-dependent characteristics. A first terminal P1 to which this load 170 is connected has a main matching block 171 connected to it. The other end (point A) of main matching block 171 is connected to one end of a delay circuit 172 having a certain reactance value. The other end (point B) of delay circuit 172 is connected to a signal source 173 having an impedance Z0 (below, the impedance not changing with frequency is called Z0).
Main matching block 171 is designed to match the impedance ZL(f1) of load 170 with the impedance Z0 of signal source 173, in frequency band f1. In other words, main matching block 171 becomes a matching circuit with respect to frequency f1. Delay circuit 172 is constituted by a distributed-parameter element, the characteristic impedance of which is given, as is well known, by the relationship shown in Eq. 1.Z0=√{square root over (L/C)}  (1)
Here, L is the inductance of the distributed-parameter element and C is the capacitance of the distributed-parameter element. Consequently, by taking the characteristic impedance of delay circuit 172 to be Z0, matching is obtained in frequency band f1 between signal source 173 and load 170.
When operating in a frequency band f2, different from frequency band f1 (e.g. when frequency band f2 is lower than frequency band f1), the impedance of load 170 changes to ZL(f2). Also, since main matching block 171 is a matching circuit with respect to frequency f1, matching between signal source 173 and load 170 is not obtained at frequency f2. Accordingly, an auxiliary matching block 175 is connected via switch element 174 to point B. And then, when operating in frequency band f2, switch element 174 is taken to be in a conducting state. By choosing a configuration like this, it is possible, whichever is the value of the impedance estimated from point A toward the side of load 170, to make the impedance Z0, seen from point B toward the side of delay circuit 172. Here, the delay value of delay circuit 172 is set to the delay value required to match at point B in frequency band f2.
With the same approach as for the matching circuit shown in FIG. 3, an example where the number of frequency bands which can be handled has been increased to three is shown in FIG. 4. By the fact that the number of frequency bands has increased from two to three, the system increases by one additional set, the set of delay circuit 180, switch element 181, and auxiliary matching block 182. In a third frequency band f3, the impedance ZL(f3) of load 170 is regulated by means of delay circuit 180 and auxiliary matching block 182 so that the impedance seen from point C toward the side of delay circuit 180 becomes Z0. Further, since the characteristic impedances of the delay circuits are fixed and do not depend on the frequency, it is possible to obtain matching between signal source 173 and load 170 in each frequency band if switch element 174 and switch element 181 are chosen to be in a non-conducting state in the case of frequency band f1, switch element 174 is chosen to be in a conducting state for in the case of frequency band f2, and switch element 181 is chosen to be in a conducting state in the case of frequency band f3.
In this way, by providing auxiliary matching blocks connected via switch elements between the delay circuits along with connecting in series in multiple stages delay circuits whose impedances do not vary with frequency, there is implemented a matching circuit capable of matching with respect to a plurality of frequency bands. Further, the delay value required in frequency band f3 can be considered to be the sum of the values for delay circuit 172 and delay circuit 180.
As for delay circuits 172 and 180, it is realistic to choose them to be transmission lines which are distributed parameter networks. However, particularly in cases where the frequency is low, transmission lines become comparatively large components inside the circuit. E.g., if load 170 is taken to be a FET and in case an amplifier for the 1 GHz band is designed, a 50 Ω transmission line has a width of 0.63 mm and a length of 9.22 mm, so the result is a component having a length of about 10 mm.
In the technology shown in the aforementioned Reference 2, the delay circuits are realistically constituted by transmission lines. However, in the case of transmission lines, the length easily becomes comparatively long. In particular, in the case where the used frequency is low, the area of a transmission line serving as a delay circuit becomes large, so there has been the problem that the matching circuit as a whole also was made bigger. Further, this problem increases as the frequency becomes lower, and as the number of frequencies rises.